Process for the distillation of a multicomponent mixture



Oct. 20, 1959 F. J. ZUIDERWEG 2,909,484

PROCESS FOR THE DISTILLATION OF A MULTI-COMPONENT MIXTURE Filed A ril10, 1957 SEPARATOR AUXILIARY DISTILLATION if COLUMN 6 I MAIN v IDISTILLATION SIDE ZONE STREAM STRIPPER BOTTOMS REBOILER INVENTOR:

FREDERIK J. ZUIDERWEG HIS ATTORNEY United States Patent PROCESS FOR THEDISTILLATION OF A 1 MULTICOMPONENT MIXTURE Frederik J. Zuiderweg,Amsterdam, Netherlands, assignor to Shell Development Company, New York,N.Y., a corporation of Delaware Application April 10, 1957, Serial No.652,049

Claims priority, application Netherlands May 24, 1956 Claims. c1. s s47thereof, and particularly in distilling ofr" lighter hydrocarbons suchas gasoline, naphtha, kerosene and gas oil, the standardized A.S.T.M.distillation method is usually employed in practice as the specificationmethod for the volatility limits of the distillates produced. With thismethod the final boiling point of the product obtained is, inter alia,determined. In the assessment of the separation by means of distillationthis temperature is very important. It is also import-ant to be able tosupply continuously a product of'which the final boiling point alwayshas a constant or practically constant value.

. There is no direct correlation between the top temperature of thedistillation column in which the mixture is treated and the finalboiling point of the distillate of this column. The top temperature ofthe column which is usually measured cannot therefore be used as ameasure of the final boiling point, so that it is usually necessary torepeat the A.S.T.M. method (which is a batchwise distillation)continuously at short intervals in order to be i t surethat thedistillate from the column shows no variations. The same applies also toa side stream of the distillation column.

It is an object of this invention to provide a process whereby the finalboiling point of a product of a fractional distillation or of themagnitude of a property directly correlated therewith is continuouslymeasured, rendering it unnecessary to continually repeat the batchwiseA.S.T.M. method. It is a further object of this invention to use thisfinal boiling point or correlated magnitude to control the distillationof the mixture in the main column in such a way that a product having a.constant or approximately constant final boiling point is obtained.

These objects are attained by continuously passing a portion of thedistillate or of a side stream of the main distillation column in liquidform into the top part of an auxiliary distillation column, the bottomof which is heated in such a way that the whole quantity of liquidintroduced is evaporated and leaves the top of the auxiliary column inthe form of vapor (except for small quantities of contaminations whichdo not evaporate, or only with dilficulty); the temperature is thenmeasured at a point between the top and bottom of the auxiliary column,and by means of this temperature one or more of the variables of thedistillation process in the main column are controlled in such a waythat the temperature measured shows, a content, or approximatelyconstant value.

5 The auxiliary column used usually has a theoretical tray number ofapproximately 4 or higher, preferably 6-8.

- The ratio between the quantity of mixture present in "the auxiliarycolumn and the quantity of liquid supplied 10.to the column per unit oftime is kept relatively low;

' preferably it does not exceed the value of 5 (expressed in minutes).In the latter case an indication of changes occurring in the main columnis obtained within 5 min- .utes.

l5 7 According to the invention there is a specific point between thetop and bottom of the auxiliary column where ,the temperature measuredis equal or approximately equal to the final boiling point of thedistillate or side .stream being controlled, as determined by theso-called 20 ..A.S.T.M. distillation method. The exact place for thispoint can be found by calibration. Once this point has .been determinedthe said final boiling point for each mix- .ture introduced into theauxiliary column can be immediately read oil. In this way this finalboiling point Y can be continuously determined. In the present controlmethod this point, corresponding to the final boiling point of themixture being controlled, is preferably selected as the measuring pointof the temperature in the auxiliary column. I

The invention is generally applicable to multi-component mixtures suchas hydrocarbon mixtures, mixtures of chlorinated hydrocarbons, alcoholmixtures, mixtures lobtained in synthesis processes such as the Fischer-Tropsch process, etc. The method is particularly suitable for theabove-mentioned mixtures of lighter hydrocarbons.

, The invention will'now be described in greater detail with referenceto the drawing in which the sole figure is a schematic representation ofa process for the control 49 ,of a continuous'distillation of ahydrocarbon mixture.

In the column 1, which is equipped with a plurality of trays, the feedis introduced approximately half-way lup via a line 2. At the top,gasoline in vapor form is drawn olf at 3 and is condensed in a condenser4. The

Q gasoline may be separated in a separator 5 from any water which mayhave distilled over. The water is drawn off at the bottom part of theseparator, while gases may be drawn off from the top. The gasolineproduct which 50 is obtained is led through a line 6 to a storage tank,not shown in the drawing. A portion of the gasoline is recycled tocolumn 1 as reflmr.

Heavier hydrocarbon fractions, such as kerosene and gas oil, may betaken from the column 1 at 7 and 8 as side'streams. These streams,preferably after heating by heat exchange with the hot bottom product ofcolumn 1, may be led to strippers and there stripped from their lightestcomponents by means of steam and/ or reboiling; the resultant vapors areagain introduced into column 1 at a suitable point. This procedure isshown in the drawing in connection with side stream 7. This stream isled via a heat exchanger 9 to stripper 10 which is equipped withreboiling means. The vapors formed are recycled to column 1 via a line11. The bottom product of the stripper is withdrawn through a line 12.

kerosene fractions with different boiling ranges. The quantity of liquidcontinuously introduced into the auxiliary column varied between 0.8 and1.9 litres per hour.

Table [Temperatures in 0.]

passed off through a line and withdrawn via the heat Final boilingpoint, A.S.T.M.. 84 101 107 110 111 135 137 142 155 164 287 301 Initialboiling point, A.S.T.M 57 81 73 65 80 76 80 77 46 57 132 220 Boilingrange 27 34 45 31 59 57 65 109 107 155 81 Temperature measured accordingto invention 85 98 106 110 110 136 138 138 158 168 280 340 The tableincludes the values of the final boiling point 15 and the initialboiling point, both determined by the exchanger 9.

According to the invention, a small portion of the condensed distillateis continuously introduced into the top part of an auxiliary column 16.This auxiliary column -may, for example, be a distillation column of theVigreux type with a length of 110 cm. and a diameter of 2.5 cm. Thecolumn is heated at the bottom part so that all liquid evaporates. Theheating may be effected, for example, electrically, as shown in thedrawing, or it can be accomplished in other ways as, for instance, bymeans of the hot residue from the main column. The vapors leave the topof the auxiliary column through a line 17 and may be cooled andcondensed to liquid outside the column. A further line with a valve (notshown in the drawing) is provided on the bottom part of the auxiliarycolumn in order to afford the possibility of drawing off at varioustimes heavy components which cannot be evaporated or are evaporated onlywith difiiculty. Such components, e.g. polymeric products, aresubstantially absent in the top product. The temperature inside thecolumn is measured at a point 18 between the top and bottom of theauxiliary column 16. The temperature value measured is transmitted to atemperature controller 19 which automatically controls in a known mannerone of the variables of the distillation process in the main column 1 insuch a way that the temperature measured at 18 shows a constant orsubstantially constant value. The degree of reboiling is shown in thedrawing as the variable of the distillation process; thus, temperaturecontroller reduces the degree of heat supplied to the reboiler 13 whenthe temperature at 18 shows a tendency to rise, and the heat supply isincreased when this temperature begins to fall.

The temperature controller could, however, have affected anothervariable just as well, e.g. the reflux of column 1; in this case thequantity of reflux should increase as the temperature measuredincreases, and decrease as the temperature decreases. The temperaturecontroller may also be made to affect more than one variable at the sametime. Moreover, instead of the distillate, a portion .of one of the sidestreams of the main column could have been led into the auxiliarycolumn.

The temperature measured at 18 is correlated with the final boilingpoint determined according to the A.S.T distillation method. It is,however, possible to indicate a point in the auxiliary column 16 atwhich the temperature is equal or almost equal to this final boilingpoint. This temperature is preferably used for controlling the maincolumn. In the above-mentioned embodiment of the auxiliary column thispoint was 5 cm. above the bottom of the column. The agreement betweenthe temperature measured at this point of the column and the finalboiling point determined according to the A.S.T.M. distillation methodis shown by the following tests which were carried out with twelvegasoline and A.S.T.M. distillation method, as well as the boiling rangeand the temperature measured in the manner according to the invention.It follows from these values that with the use of the present processthe final boiling point according to the A.S.T.M. method can, in fact,be determined continuously.

I claim as my invention: 1

l. A method of controlling a fractional distillation process wherein amultiacomponent feed is evaporated in a main vertical distillation zoneand various relatively narrow fractions of the feed are withdrawn fromdifferent parts of said main distillation zone, the heavier fractionsbeing withdrawn toward the bottom and the lighter fractions toward thetop, which comprises transferring a small portion of a fraction inliquid form from said main distillation zone to the top of a relativelysmall auxiliary vertical distillation zone, supplying heat at the bottomof said auxiliary zone such that substantially all of said portion isevaporated as it passes downward in said auxiliary zone and the vaporstherefrom rise up through downcoming liquid in said auxiliary zone, andusing the temperature at one point intermediate the top and bottom ofsaid auxiliary zone at which point the highest boiling fractions arevaporizing to control at least one of the variables relating to thethermal balance of said fractional distillation process such that .thefinal boiling point of said fraction is determined according to apredetermined correlation between said temperature and said finalboiling point.

2. The method of claim 1 wherein the point of the auxiliary distillationzone at which the temperature is used as a control is determined suchthat said temperature is approximately equal to the final boiling pointof said fraction being controlled.

3. The method of claim 1 wherein the multi-cornponent feed is ahydrocarbon mixture obtained from a crude .oil.

4. The method of claim 1 wherein the vapors leaving the top of the maindistillation zone is condensed and a portion of the condensate is theliquid which is evaporated in the auxiliary zone in order to control thefinal boiling point of said vapors.

5. The method of claim 3 wherein gasoline vapors leaving the top of themain distillation zone are condensed and a portion of the resultinggasoline is trans ferred to the auxiliary zone in order to control thefinal boiling point of said gasoline.

References Cited in the file of this patent UNITED STATES PATENTS2,104,310 Roelfsema Jan. 4, 1938 2,315,122 Kraft Mar. 20, 1943 2,340,026Storment Ian. 25, 1944 2,580,651 Boyd Jan. 1, 1952

1. A METHOD OF CONTROLLING A FRACTIONAL DISTILLATION PROCESS WHEREIN AMULTI-COMPONENT FEED IS EVAPORATED IN A MAIN VERTICAL DISTILLATION ZONEAND VARIOUS RELATIVELY NARROW FRACTIONS OF THE FEED ARE WITHDRAWN FROMDIFFERENT PARTS OF SAID MAIN DISTILLAION ZONE, THE HEAVIER FRACTIONSBEING WITHDRAWN TOWARD THE BOTTOM AND THE LIGHTER FRACTIONS TOWARD THETOP, WHICH COMPRISES TRANSFERRING A SMALL PORTION OF A FRACTION INLIQUID FORM FROM SAID MAIN DISTILLATION ZONE TO THE TOP OF A RELATIVELYSMALL AUXILIARY VERTICAL DISTILLATION ZONE, SUPPLYING HEAT AT THE BOTTOMOF SAID AUXILIARY ZONE SUCH THAT SUBSTANTIALLY ALL OF SAID PORTION ISEVAPORATED AS IT PASSES DOWNWARD IN SAID AUXILIARY ZONE AND THE VAPORSTHEREFROM RISE UP THROUGH DOWNCONING LIQUID IN SAID AUXILIARY ZONE, ANDUSING THE TEMPERATURE AT ONE POINT INTERMEDIATE THE TOP AND BOTTOM OFSAID AUXILIARY ZONE AT WHICH POINT THE HIGHEST BOILING FRACTIONS AREVAPORIZING TO CONTROL AT LEAST ONE OF THE VARIABLES RELATING TO THETHERMAL BALANCE OF SAID FRACTIONAL DISTILLATION PROCESS SUCH THAT THEFINAL BOILING POINT OF SAID FRACTION IS DETERMINED ACCORDING TO APREDETERMINED CORRELATION BETWEEN SAID TEMPERATURE AND SAID FINALBOILING POINT.